1 circuit with many different types of LED's judging resistances for all to be bright

Discussion in 'General Electronics Chat' started by sirchuck, Mar 20, 2016.

  1. sirchuck

    Thread Starter Member

    Feb 14, 2016
    54
    2
    R = V / I can tell me what resistor to use for one LED, but I have different types of LED's each with different resistances. For my example I'll use one purple and one blue. When I put the purple one in the circuit the blue one goes out because there is less resistance through the purple LED I assume.

    So i'm wondering how would I set up a circuit to handle different colored LED's, and keep them all bright. I noticed I can add a resistor to the purple one to let the blue one and purple one light, but how do you figure out the proper resistors for each LED?

    Thanks!
     
  2. Picbuster

    Member

    Dec 2, 2013
    376
    50
    Look at the factory specs when not available you have to measure the light at different currents or observe visually.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    Each led should have its own resistor.
    The resistor can be calculated as followed:
    R = (Vsupply - Vled_voltagedrop)/Iwanted.

    Bertus
     
    Pronay likes this.
  4. Picbuster

    Member

    Dec 2, 2013
    376
    50
    Correct Bertus however; the amount of light produced will differ per colour. (Red @ 20mA will produce different amount of light then blue @20mA) This will force you to adjust each resistor. making your own white light with 3 leds will accentuate this.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    I know. Always look at the datasheet. There can be a graph about the relative amount of light output versus the current.
    Something like this:

    luminous_flux_versus_forward_current.png

    This comes from the attached datasheet of the VLWW9900 led.

    Bertus
     
  6. sirchuck

    Thread Starter Member

    Feb 14, 2016
    54
    2
    Ok, so if I understand you guys are telling me to just go ahead and use the R = V/I equation for each LED / Resistor pair placed in parallel on my circuit. Is that correct?

    So when wiring in parallel do the math the same way between each resistor and LED on the circuit.

    Thanks, I was thinking each new LED and Resistor set would change the math somehow like lower the supply voltage for the next piece of the circuit.
     
  7. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Only if you have a cheap power supply.
    One long line with a bunch of resistors attached, each resistor goes to one LED, adjust resistance if necessary.
    I would like to say making a drawer full of random LEDs match in intensity is a fools errand. Leds have been getting about twice as much light every 20 years for a long time. If you want to match them you need similar LEDs.
     
  8. dannyf

    Well-Known Member

    Sep 13, 2015
    1,828
    365
    1. Find the desired lumens and corresponding forward current to generate that lumen.
    2. Find the corresponding voltage drop under that forward current.
    3. Calculate the required resistance.
    4. Set back and enjoy your work.
     
  9. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    When I need to adjust relative luminosity of a handful of LEDs to match brightness of several colors or get a specific color mix from an RGB, I breadboard them side by side with the correct resistor for maximum allowable current for each, identify the dimmest and use a low value potentiometer in series with each of the others to dial the brightness I want. Then I measure the combined resistor+pot value and look for a suitable single replacement resistor.
     
  10. sirchuck

    Thread Starter Member

    Feb 14, 2016
    54
    2
    Thanks for the advice. I don't really need all the luminosity to match, just wanted to be able to see all the lights at their individual normal brightness.

    I do have a potentiometer handy, I'll use that suggestion to fine tune.
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
    1,964
    219
    Yes, one resistor for each LED. Forget the calculations. Select the resistors for the desired brightness.
     
Loading...